Description and Use of Municipal Solid Waste Composts in New Mexico

Circular 562

College of Agriculture, Consumer and Environmental Sciences New Mexico State University

This Publication is scheduled to be updated and reissued 6/04.

The average American generates approximately 4.5 pounds of garbage
per day, for a total of 196 million tons of trash per year, most of which
ends up in landfills (Shortridge, 1993). Various government and private
programs have been organized to divert part of this garbage (metals, plastics,
paper and cardboard, and yard and food wastes) from landfills by recycling.
Yard and food wastes often are recycled as compost.

A 1991-1992 landfill survey showed that yard waste made up 33.8 percent
of the City of Albuquerque's residential solid waste stream (Romo, Cave,
and Watkins, 1992). During the same period, the city's wastewater treatment
facility was treating more than 50 million gallons of mixed residential
and industrial wastewater daily, producing about 22 dry tons of stabilized
biosolids (sludge) per day.

To address these solid waste problems, the City of Albuquerque dedicated
a state-of-the-art municipal compost facility in early 1992. Since then,
portions of the source-separated landscape wastes (2,500 ton/year of yard
trimmings) have been combined with biosolids (9,700 ton/year) and horse
track manure (3,600 ton/year) at the facility to create an EPA-certified
(Class A) biosolid compost (Glass, 1997). In 1996, the same facility began
making a green waste compost using urea (fertilizer) instead of biosolids
as a nitrogen source. Other municipal composting operations in New Mexico
are located at Artesia, Los Alamos, Alamogordo, Carlsbad, and Tucumcari
(Baker, 1998). A number of private companies compost yard wastes, and permits
are pending for other municipal composting programs.

COMPOSTING VERSUS LANDFILLING

Yard wastes and biosolids not only occupy valuable space in landfills,
but they also decompose, which can result in the production of methane
gas and leachates that pollute the environment. Also, collection of yard
waste is expensive. Recycling these organic wastes as composts benefits
the environment, and financial returns from the sale of municipal composts
can help offset the costs of collection and processing. Recycling the composts
back on city parks, in local gardens, on farms, or for revegetation of
disturbed lands, also can make such entities more sustainable.

THE COMPOSTING PROCESS

Composting is the aerobic, biological degradation and oxidation of organic
wastes such as manures, biosolids, food scraps, and yard trimmings by various
naturally occurring microorganisms under controlled conditions which results
in a stabilized, humus-like material.

The criteria for "stabilized"
compost vary somewhat, but in general, a compost is considered stable when
the temperature in a static pile remains at or near ambient air temperatures
for several days; moisture content is about 50 percent; and oxygen content
is more than 5 percent.

Municipal solid waste composting is composting on a community-wide scale.
Most municipal solid waste composting facilities in New Mexico are source-separated
operations where yard wastes are separated from other wastes such as cans
and plastic. Biosolid composts are co-composted products where biosolids
are combined with yard wastes. Green waste composts generally only involve
yard wastes and the possible addition of a nitrogen fertilizer.

Optimum composting conditions involve a balance of six factors: (1)
the carbon:nitrogen ratios of feedstocks, (2) particle size, (3) oxygen,
(4) moisture, (5) temperature, and (6) time. The composting process involves
the generation of heat, production of carbon dioxide, loss of water vapor,
loss of mass (waste), and production of a relatively stable humus that
is free of offensive odors.

All organic wastes (feedstocks) are made of carbon and nitrogen. When
combining various organic wastes in a composting operation, the ideal ratio
of carbon to nitrogen is 30:1 for optimum growth of bacteria, which dominate
the initial composting process. Microorganisms like bacteria require carbon
for energy and nitrogen for the production of amino acids and proteins
in their bodies. Higher carbon:nitrogen ratios slow the composting process,
while lower carbon:nitrogen ratios produce too much ammonia. Raw materials
high in carbon (such as sawdust, leaves, and paper) should be combined
with raw materials high in nitrogen (such as manure, grass clippings, and
food waste) to obtain the appropriate 30:1 ratio for optimum composting
conditions.

Feedstocks used in composting often are shredded or ground to increase
the surface area of these materials. The particle size of these materials
after processing affects the porosity of the compost, which in turn affects
the flow of air in the compost. The smaller the particle, the greater the
surface area and the faster the compost will decompose. If the particle
is too small, however, airflow is inhibited. If the particles are too large,
the compost dries out. Best results are obtained when the particle ranges
from 1/8 inch to 2 inches in diameter.

Compost should contain at least 5 percent or more oxygen for optimum
aerobic composting. Aeration in most New Mexico municipal composting operations
is passive, such that oxygen enters the compost by natural convection.
Turning of a compost pile or windrow provides only limited aeration. Good
porosity is important for natural convection and diffusion of oxygen into
the compost. Insufficient oxygen results in anaerobic conditions and the
production of objectionable odors (from chemicals such as hydrogen sulfide,
methane, and organic acids).

Compost should contain 40 to 60 percent moisture to support the growth
of microorganisms involved in the composting process. Microbial activity
is severely inhibited below this range. Moisture levels above 60 percent
result in anaerobic conditions.

The breakdown of wastes by bacteria in the composting process generates
heat. Mesophilic organisms generally are active at temperatures of 70°
to 105°F. Thermophilic bacteria take over at higher temperatures (110°
to 150°F). At temperatures above 160°F, microorganism activity
begins to shut down. Turning piles mechanically at this stage helps bring
the temperature down. Compost on the outside of a pile or windrow should
be turned to the middle of the pile so it can undergo the composting process.
Temperatures are considered ideal between 131° and 150°F, which
kill most weed seed and pathogens (both plant and animal). The compost
should be kept at these temperatures for as long as possible.

As the organic food supply is depleted, the compost begins to cool.
Heat, water vapor, and carbon dioxide released during the composting process
reduce the overall size of the compost pile or windrow by as much as 50
percent. When the compost temperature drops to 100°F or less and fails
to reheat after turning, the compost is allowed to cure for one to six
months or more under natural aerobic conditions. During the curing process,
various fungi and actinomycetes form. Actinomycetes form filaments like
fungi but are much smaller, so they are classified as higher forms of bacteria.
Both fungi and actinomycetes tend to feed on more resistant materials,
such as cellulose and lignins, that are left over after the composting
process. Actinomycetes also convert volatile organic acids into longer
chained organic acid complexes (5 carbon ringed humic acids), which make
up humus. This tends to stabilize the nutrients in the compost. Curing
is considered complete when the pile remains at or near ambient air temperatures
and the respiration rate (rate of oxygen consumed) is less than 200 mg
O2 per kg of compost per hour. The compost can then be screened for various
agronomic and horticultural uses.

Biosolid Composting Process

Composting of biosolids or sludge is regulated by the Environmental
Protection Agency (USEPA, 1993). Heavy metals in biosolid composts must
be in compliance with the Clean Water Act of 1987, 40CFR, Part 503, for
"exceptional quality" (table 1). To date, the City of Albuquerque's Municipal
Compost Facility has met these requirements as well as EPA's requirements
for polychlorinated biphenyl (PCB) content (Glass, 1997). To control pathogens
in the compost, windrows of composted biosolids and yard waste are kept
at 55°C (131°F) for 15 days and turned 5 times (every 3 days) during
the maximum heating phase. The finished compost (Class A) should have less
than 1000 organisms (most probable number) per gram of total dry solids
for the benign indicator bacteria, fecal coliform (USEPA, 1993).

1Soil, Water, and Air Testing
Laboratory, NMSU.
2LT = Less Than 1 mg/kg

The production of green waste compost (from yard trimmings) is somewhat
less expensive than that of biosolid compost because fewer state and federal
regulations apply. Urea is often added as a supplemental nitrogen source
for this type of composting.

CHARACTERSITICS OF COMPOST

The specific characteristics of various composts depend on the organic
wastes being composted and the composting process. The ultimate use and
value of any compost depends not only on its physical and chemical properties,
but also on its biological activity.

Physical Characteristics

Compost particle size depends on how the feedstocks were processed (chipped,
shredded, or ground), the thoroughness of the composting process, and whether
the compost is screened after curing. Particle size affects the ultimate
bulk density (lb/yd3) of the finished compost (average is between 800 lb/yd3
and 1000 lb/yd3), which in turn affects the structure of the soil it's
applied to. Smaller particles increase the surface area of a compost, thus
increasing the water-holding capacity of a soil (particularly important
in sandy soils) and its ability to retain nutrients (cation exchange capacity).
Composts with larger particles have lower bulk densities and are used as
mulches or in heavy soils to increase aeration and improve water infiltration
(by enhancing soil aggregation). Composts vary in organic matter content
from 30 to 70 percent, but the ideal is more than 50 percent.

Moisture content of finished composts should range from 45 to 50 percent.
Composts with less moisture tend to blow in the wind, while those with
more moisture are heavier and tend to clump.

Composts should be free of metal, glass, plastic, cement, asphalt, and
other debris (no greater than 1 percent). Most debris can be controlled
with a good source-separation program.

Chemical Characteristics

The nutrient content of most composts is relatively low compared to
most commercial fertilizers. Composts containing manures or biosolids tend
to contain more nutrients than pure green waste composts. Although composts
are relatively low in macronutrients (nitrogen, phosphorous, and potassium),
they generally are an excellent source of micronutrients (such as iron,
zinc, and magnesium), especially composts made from biosolids. As most
nutrients in compost are in organic forms, they are slowly made available
for plant uptake, thus they are considered more environmentally friendly.
Biosolid composts, however, often contain low levels of undesirable heavy
metals like lead, mercury, arsenic, and cadmium.

Soluble salts in a compost include many of the above nutrients as well
as salts like sodium. Soluble salt content is generally expressed as the
electrical conductivity (measurement of the readiness of a medium to transmit
electricity) of the compost or soil. Electrical conductivity is generally
expressed in millimhos per centimeter (mmho/cm) at 25°C. Some crops,
like green beans, are very sensitive to salts, while others, like asparagus,
are more tolerant. The ideal compost should have an electrical conductivity
no higher than 3.5 mmho/cm.

The pH of a compost can affect how soluble the nutrients will be in
a plant's root zone. Some greenhouse plants and strawberries prefer acidic
soils, while many turfgrasses tolerant more alkaline soils. Most composts
have a pH of 6.8 to 7.5 (Naylor, 1993).

The carbon:nitrogen ratio of a compost is important because it is a
general indicator of whether a compost will rob plant roots of nitrogen.
The ideal finished compost should have a carbon:nitrogen ratio between
15:1 and 25:1 (Naylor, 1993). Higher carbon:nitrogen ratio composts can
be used in crop production, but they may need to be supplemented with nitrogen
fertilizer.

Compost maturity relates to the stability of the organic mass. Immature
composts can contain organic acids, ammonia, and other phytotoxic chemicals
that can damage plant roots. For the compost to mature and stabilize, sufficient
time must be allotted for proper curing of compost under aerobic conditions.

Compost also should be free of objectionable odors and have a pleasant
appearance. Ideal composts have an earthy smell and a dark, crumbly, humus-like
appearance.

Biological Characteristics

Unlike most peat mosses, compost is biologically active. Microorganisms
in compost continue to break down the various complex carbonous materials
in the compost, slowly making nutrients available for plant uptake.

Many microorganisms that recolonize the compost during the curing phase
can behave like biological fungicides by controlling various soil-borne
plant diseases. Composts, particularly those made from tree bark, have
been used for many years in the nursery industry as a substitute for peat
moss. Tree bark composts have been found to suppress soil-borne diseases
caused by Fusarium spp., Phytophthora spp., Pythium spp., Rhizoctonia solani,
and other pathogens (Hoitink and Grebus, 1995). The mechanisms for biological
control are thought to be based on competition, antibiosis, hyperparasitism,
and induced systemic resistance in plants (Hoitink, 1993). Under- and overmature
composts, and those high in soluble salts have less fungicidal effect (Rynk,
1992).

USES OF COMPOST

Municipalities often find composting an environmentally preferred alternative
to landfilling yard wastes and biosolids. Composting not only avoids methane
and leachate production in the landfill, it also saves valuable landfill
space. Compost production also can generate funds that can be returned
to municipal governments through the direct sale of compost to the public
or by reducing the purchase of fertilizers and soil amendments used on
city parks, golf courses, cemeteries, and other properties.

The major benefit of compost is to improve the structure of a soil.
It is also a source of plant nutrients, can be used as a mulch, and, in
some cases, can be used as a natural fungicide. The application rates of
compost depend on its characteristics and intended use.

Agricultural Cropland

Agricultural cropland is the largest potential market for compost use,
but it is one of the least profitable. Municipal compost is applied to
cropland primarily to improve the structure of a soil (such as to increase
its water-holding capacity, cation-exchange capacity, and, in heavy soils,
water infiltration and aeration). Municipal compost also is used as a source
of nutrients, particularly minor elements, because most commercial fertilizers
rarely contain minor elements. The organic matter in compost also acts
as a relatively long-term reserve for major nutrients like nitrogen, phosphorous,
and potassium. It is estimated that 10 to 15 percent of nitrogen in this
organic matter may be released during the first growing season through
mineralization, with residual nitrogen released over the next 2 to 3 years
(Naylor, 1993).

The particle size of compost applied to cropland can be 1/2 to 3/4 inch
in diameter. It is important that the carbon:nitrogen ratio be no greater
than 30:1 so that the compost does not rob plants of nitrogen (although
extra nitrogen fertilizer can be applied to overcome this tendency). The
compost also must be mature to prevent damage to plant roots from excess
ammonia, organic acids, and other phytotoxic chemicals. Immature composts
can be applied directly to cropland if it's done well in advance of planting
to give the compost a chance to stabilize in the soil. For example, for
spring-planted crops, apply the compost to the soil the fall before planting.

The application rate of municipal compost basically depends on its nutrient
content, soluble salt concentration, soil reserves of nutrients as determined
by soil testing, and the crop to be planted. The application rate for most
food crops should not exceed 50 dry tons per acre (Rynk, 1992). Generally,
optimum rates are 10 to 20 tons per acre, but rates depend on soil test
results and crop requirements.

A municipal compost's application rate, soluble salt concentration,
and ability to suppress disease in combination with irrigation technique
can affect yields. In 1995, 5 rates (3 replications) of a biosolid compost
(EC = 14.5 mmho/cm) from the City of Albuquerque, were applied to a chile
field in Garfield, New Mexico; disked; listed; pre-irrigated; and planted
to "Sandia" chile. After removing the soil cap (a layer of soil over the
seed to prevent soil crusting) from the bed after the seed germinated,
seedling counts were made at various times during the season beginning
on 5/17/95 (table 2).

There was a 27.2 percent mortality rate in seedlings in the check plot
compared with the 50 ton/acre plot (5/17). These losses were attributed
to "damping off" (Rhizoctonia solani) due to cool temperatures early in
the growing season. These results, however, were reversed by the end of
the season (10/2), with the greatest losses occurring in the 50 ton/acre
plot. These losses were attributed to salt and damping off. Losses to chile
wilt (Phytophthora capsici) also were heaviest in the 30 ton/acre and 50
ton/acre plots. The 20 ton/acre plots resulted in the lowest losses from
either damping off or chile wilt and produced the greatest yields. Although
the compost suppressed both diseases, the high content of soluble salt
in the biosolid compost at 30 ton/acre and 50 ton/acre resulted in greater
plant losses from both diseases. The salt concentration was aggravated
by the fact that the plants were in the middle of the bed, where salts
tend to accumulate. Higher compost rates may be tolerated when plants are
established on the outside edge of a flat vegetable bed (because salts
tend to move to the middle of the bed by capillary action), are sprinkle-irrigated
(so salts leach through soil profile), or are placed under drip irrigation
next to plants so that salts are moved away from the plants.

There is some concern that heavy metals can accumulate in the soil from
heavy applications of biosolid compost. This was not the case in the chile
experiment (table 3). Analyses of soil samples (from an 8-inch depth) taken
at the end of the growing season from each plot showed only random differences
in metal contents at all rates. The high iron figures may have been due
to lab error or a fertilizer application. Where food crops are grown in
fields treated with biosolid composts, the heavy metal content of fields
should be monitored yearly.

Table 3. Effects of biosolid compost on soil nutrients and
other parameters after one growing season (chile), 11/95.

Parameter

0

Biosolid 10

compost 20

(ton/acre)30

50

pH

7.94

7.46

7.63

7.67

7.78

EC (mmho/cm)

1.93

2.85

1.81

2.35

1.61

Organic matter (%)

1.18

1.57

1.54

1.71

1.48

Nitrate nitrogen (ppm)

18.6

62.6

20.2

34.5

22.5

Sodium (meq/l)

9.26

10.72

8.09

10.03

7.19

Calcium (meq/l)

8.66

14.27

8.50

12.01

7.3

Magnesium (meq/l)

2.36

4.08

2.43

3.26

2.03

Phosphorous (ppm)

28.4

52.8

46.8

15.9

52.8

Potassium (ppm)

40.0

61.0

52.0

64.0

54.0

Lead (mg/kg)

17.0

17.0

17.0

14.0

17.0

Cadmium (mg/kg)

LT

LT

LT

LT

LT

Nickel (mg/kg)

7.2

6.9

7.2

6.5

7.1

Arsenic (mg/kg)

1.1

1.18

1.2

1.1

1.1

Chromium (mg/kg)

10.8

11.3

10.0

9.2

11.2

Copper (mg/kg)

12.0

13.0

14.0

13.0

14.0

Iron (mg/kg)

15,510

14,590

15,020

11,980

15,220

Mercury (mg/kg)

LT

LT

LT

LT

LT

Molybdenum
(mg/kg)

LT

LT

LT

LT

LT

Selenium (mg/kg)

LT

LT

LT

LT

LT

Silver (mg/kg)

LT

LT

1.6

LT

LT

Zinc (mg/kg)

53

58.0

56.0

46.0

53

1LT = Less Than 1.0 mg/kg

Nursery Crops

The "green industry" (greenhouses, nurseries, garden centers, landscape
contractors) makes up the second largest agricultural industry by income
in the United States. Given that about 80 percent of all marketed containerized
ornamental plants are grown in media made of 75 to 80 percent organic matter,
this industry could be a huge potential market for compost. The use of
compost in creating new topsoil also is important. It is estimated that
harvesting 1 acre of balled and burlapped trees and shrubs removes more
than 200 tons of soil. Compost often is used to create new topsoil to keep
such operations sustainable (Gouin, 1995).

While the quality of compost applied to cropland can vary widely, compost
used in the green industry must be consistent in its quality. Two of the
most important considerations in quality are compost maturity and soluble
salt content.

Composted tree bark has been evaluated for many years as a substitute
for peat moss in potting soil media. Although properly stabilized or mature
compost helps suppress soil-borne pathogens in potting media, overly stabilized
or immature composts are not as effective. Composts high in soluble salts
(particularly biosolid composts) also can reduce the effectiveness of the
compost in suppressing soil-borne diseases (Hoitink, 1993).

In the spring of 1996, four potting soil mixes were evaluated for their
ability to suppress damping off (Pythium ultimum) in a greenhouse (Edgewood,
New Mexico). The check (grower treatment) consisted of 25 percent vermiculite,
25 percent perlite, and 50 percent peat moss. The other three treatments
also consisted of 25 percent vermiculite and 25 percent perlite. A sifted
biosolid compost (EC=14.5 mmho/cm), however, was used to replace some of
the peat moss in three treatments. The treatments were 12.5 percent compost
with 37.5 percent peat moss; 25 percent compost with 25 percent peat moss;
and 50 percent compost with no peat moss. The treatments were then seeded
with snapdragons.

Higher salt concentrations from the heavier compost rates increased
the number of days to emergence for the snapdragons and decreased the emergence
rating (table 4). Heavier compost rates also decreased plant height.

On 4/3/96, healthy snapdragon plants were transplanted to new trays
containing the same media treatments. The 25-percent compost treatment
resulted in the greatest suppression of damping off (Pythium ultimum),
followed by the 12.5-percent rate (table 4). The check, with no compost
and 50 percent peat moss, resulted in the lowest suppression rating. Although
the 50 percent compost treatment showed some signs of disease suppression,
the high salt content of the compost reversed benefits of disease suppression

Most potting soil mixes range from 20 to 33 percent compost. The best
mixture depends on the plant species and the soil's soluble salt content
(Rynk, 1992). The pH of the media can be regulated, to some extent, by
the feedstocks used in creating the composts and amendments like sulfur
(which makes soil more acidic).

Given the nutrients in most composts (especially biosolid composts),
liquid fertilizers generally are not required by most potting plants for
the first couple of weeks of growth. Slow-release fertilizers may be blended
with the potting soil mixture if required.

Landscaping and Turfgrass

Municipal composts can be applied to new landscapes at a rate of up
to 50 dry ton/acre (2,296 lb/1000 ft2). Rates will vary depending on the
soluble salt content of the compost. After rough grading, the compost should
be incorporated into the existing soil to a depth of 6 to 10 inches. After
removing rocks and other debris, the site can be raked smooth and planted
to sod or other ornamentals (Gouin, 1995).

When planting shrubs and trees, some landscapers backfill individual
planting holes with a mixture of 25 to 50 percent compost and native soil.
Composts with higher soluble salt contents should be applied at lower rates.
Lower rates also should be used for native plants.

Municipal composts can be applied to flower beds and vegetable gardens
at a rate of 1000 lb/1000 ft2 to 2000 lb/1000 ft2. Lower rates should be
used if the soluble salt content is high. Rates also will vary with the
initial soluble salt content of the soil, type of crop planted, and watering
technique.

Peat moss has been a common component of top-dressing mixes used to
treat golf course greens, fairways, football fields, and parks throughout
the United States. Mixtures often include various grades of sands, zeolites
(volcanic mineral amendment), and peat moss. The peat moss may range from
15 to 20 percent by volume in such mixes.

Given the finite quantities of peat moss and its transportation cost,
peat moss can be quite expensive ($7.44/3.8 cubic feet, 1998, an Albuquerque
retail outlet). Many New Mexico turfgrass managers have replaced the peat
moss in top-dressing mixes with greater amounts of zeolites. An alternative
is to use local municipal compost to replace the peat moss. Per cubic yard,
peat moss is three times more expensive than municipal compost in Albuquerque.

Besides being less expensive than peat moss, municipal compost has an
advantage over peat moss and zeolites because it provides a wide range
of nutrients for plant growth. Because these nutrients are in organic form,
they are released slowly during the growing season with less nutrient loss
to leaching. Compost also contains humic acid, which makes other nutrients
in the soil more available for plant uptake.

As in the nursery industry, municipal compost also has been found to
be more biologically active than peat moss or zeolites when used on turf.
Many composts have been found to be effective in suppressing diseases like
brown patch, dollar spot, Pythium blight, and Pythium root rot (Tyler,
1996).

The particle size for municipal compost in top-dressing mixes should
be no larger than 1/8 to 1/4 inch in diameter, particularly on golf course
greens. Top-dressing is combined with aeration operations in the spring
and fall. Spreaders should be calibrated to apply a 1/4-inch layer of mix
that may or may not be mixed in with plugs using a drag or keystone mat.
Excess material may have to be blown off the greens if the compost particle
is larger than 1/8 of an inch. Typical topdressing mixes include 15 percent
compost, 20 percent zeolites, and 65 percent sand; or 10 to 30 percent
compost and the rest, sand.

Municipal compost mixes also are very popular in repairing divots on
golf courses and tees. Municipal composts also can be used to rebuild or
repair poor soil in rough areas, parks, and other non-USDA-specified turf
areas. Compost with a larger particle (1/2 inch) can be used in such cases
because itÕs incorporated into the soil.

Compost quality is important when considering its use in top-dressing
mixes or soil mixes. Parameters to be considered include particle size,
pH, soluble salts, nutrient content, stability of heavy metals, stability,
weed seed, phytotoxic compounds, and foreign objects. Quality also should
be relatively consistent from batch to batch to insure a consistent response
from the turf.

Compost also has been used to stabilize slopes. Compost with a larger
particle (1/2 inch) is often combined with soil stabilization mats or netting,
then over-seeded with native grasses to control erosion, especially along
highway right-of-ways.